Non-woven materials and other fibrous products consist of a loosely assembled mass of fibers that are bound together with a polymeric binder to form a self-sustaining web that can be used to produce many items such as consumer towels, disposable wipes, absorbent media for feminine hygiene applications and diapers medical drapes, table-cloths, and high-grade napkins. The strength of the non-woven fabric, especially wet tensile strength, is an important property in many applications.
One way to improve the tensile strength of a non-woven material is through the incorporation of crosslinking monomers into the polymer. The crosslinking monomers are capable of self-crosslinking after application to the non-woven web. The most widely used crosslinking monomer in such applications is N-methylol acrylamide. There are two problems with the cross-linking monomers. First, is that there is an upper limit to amount of the cross-linking monomer that can be incorporated to produce a useful binder under current processes. Second, N-methylol acrylamide is a recognized source of formaldehyde, which is undesirable in most applications. Several methods have been used to take advantage of the higher tensile strength available from the use of N-methylol acrylamide, while keeping the residual formaldehyde levels low.
U.S. Pat. No. 4,449,978 discloses the use of acrylamide to replace some of the N-methylol acrylamide(NMA). With N-methylol levels of from 1.75 to 3.5 percent of the polymer, free formaldehyde levels of below 10 ppm were obtained.
U.S. Pat. No. 5,540,987 discloses the use of an ascorbic acid initiator system to reduce the free formaldehyde levels to less than 10 ppm for a non-woven binder containing from 0.5 to 10 percent, and preferably from 1–5 percent of N-methylol acrylamide or other crosslinking monomers. Exemplified are emulsion polymers having from 3 to 5 percent of NMA, formed at a polymerization temperature of 75 to 80° C.
There is a need for a binder that can provide a non-woven fabric with a higher level of wet tensile strength than currently available. For many applications, the high wet strength must be obtainable at a low level of formaldehyde.
Surprisingly it has been found that ethylene-vinyl acetate emulsion binders having higher levels of cross-linking monomer such as n-methylol acrylamide, that are made by a low temperature polymerization, produce non-woven products having high wet tensile strength, yet have low (less than 15 ppm) of formaldehyde.